Streams of single photons, or single pairs of polarization-entangled photons, arriving within known time intervals have potential applications in the new fields of quantum communications and quantum computing. Most significantly, the recently demonstrated scheme of quantum cryptography involves encoding information on the polarization of a single photon. Security from eavesdropping is provided by the fact that one cannot measure the polarization of a single photon without altering it. Large expenditures will soon be directed towards research on quantum cryptography and efficient single photon sources.
Several protocols for quantum cryptography have been proposed, of which “BB84” in reference “C. H. Bennett and G. Brassard. Proceedings of IEEE International Conference on Computers, Systems, and Signal Processing, Bangalore, India IEEE, New York, 1984, pp. 175-179, 1984” and “Ekert” in reference “A. K. Ekert. Quantum cryptography based on Bell's theorem. Physical Review Letters, 67(6), 661-663, 1991” are two popular examples. In the BB84 protocol, a stream of single photons is needed. The presently available source, a highly attenuated laser, leaves the arrival photon number random, and thus requires a much lower data transmission rate to avoid two-photon events, than would be possible with a regulated photon source. One of the most successful methods so far to generate single photons is single-molecule fluorescence. A molecule is excited by a laser pulse, and emits a single photon in response. This approach suffers from two difficulties, rapid photo-bleaching (destruction of optical activity) of the molecules, and limited collection efficiency of the emitted photons. Another approach is fluorescence from single color center in diamond crystals. However, in this approach the emitted photons have a very large variation in wavelength. In the Ekert protocol, a stream of polarization-entangled photon pairs is needed. The most practical existing entangled photon source, parametric down-conversion, produces a number of photon pairs according to a Poisson distribution, rather than deterministically producing exactly one photon pair. A source of single pairs of polarization-entangled photons would be beneficial to this scheme. A few other applications for such photon sources are possible, including a random number generator, a light intensity standard, fundamental tests of quantum mechanics, quantum teleportation and quantum computation.
Accordingly, there is a need to develop a device and method that is capable of providing both triggered single photons of definite polarization, and triggered pairs of polarization-entangled photons. In addition, there is a need to develop a device and method that does not suffer from photo bleaching, and allows a high collection efficiency.